For me, vigilante movies are always a good idea, and when done right, send a resonance through people. V for Vendetta is one of those. It's portrayal of corrupt government and the power of the people was great, but even more than that, it's depiction of the people's accountability and the price of freedom was amazing
For the Boondock Saints, it's a similar thing. Circumnavigating the law to do what is right and necessary. What I like about the movie is that it's not only well done, but hilarious as well. The characters and idea is sold to us even though it's pretty unrealistic. And in spite of the lack of realism, we BUY it. Then there's the religious backing of their actions. Most vigilante movies (vendetta, Batman, etc) the hero(s) become crime fighters because of some personal loss. With the Saints, it's a divine calling that, unknown to them, reaches back through their blood lines. Even if you're an athiest, you gotta appreciate the level of accidental competence they posess. Then there's the prayer they recite before they ice somebody (as seen in my sig) The prayer isn't just badass like when Jules in Pulp Fiction does one. It has a purpose. It's acknowledging last rights of the victim and dedicating their actions to God and man. It's putting a purpose to the kill. They see themselves as the avenging hammer of God and who can refute the evidence.
All in all though, it's just a fun movie. It's so off kilter and different from the norm that you can't help but like it in some way or another. For me, I was late to the movie as well. Saw it for the first time two years ago, and after one viewing, I liked it, but wasn't blown away. The more I thought about it, the more it grew on me to where it's one of my favorites now.